In an internal combustion engine, a crankshaft may be used to convert the reciprocating motion of the pistons into rotational motion. The crankshaft may include, offset from its rotational axis, a plurality of crankpins. Each crankpin is coupled through a bearing to a piston rod, which is linked to a corresponding piston. The crankshaft itself is supported by two or more main bearings, in which it rotates.
To prevent excessive vibration while rotating, the crankshaft may include a series of counterweights that counterbalance the mass of each crankpin, associated piston, piston rod, and entrained lubricant. The counterweights may prevent excessive vibration of a crankshaft perpendicular to its rotational axis, but do not address the torsional vibration that may occur about the rotational axis. More specifically, each piston rod transmits a torsional impulse to its attached crankpin during the power stroke of the associated piston. With each torsional impulse received, the crankshaft twists slightly about its rotational axis, then twists back after the power stroke. In this manner, periodic torsional impulses from each of the piston rods may drive a complex torsional vibration in the crankshaft. Depending on conditions, such vibration may coincide with an order of a natural resonance frequency of the crankshaft. When this occurs, the vibration may increase in amplitude, such that the crankshaft is inelastically deformed, causing material failure.
Certain crankshaft components may be used to suppress torsional vibration and thereby protect the crankshaft from failure—flywheels and torsional dampeners, for example. Another approach is to install one or more so-called ‘pendulum absorbers’ on the crankshaft. A pendulum absorber is a mass non-rigidly linked to the crankshaft at a predetermined distance from the rotational axis of the crankshaft. It reduces torsional stress in the crankshaft by absorbing one or more crankshaft firing-order rigid-body modes. When the crankshaft receives a torsional impulse at a crankpin, that impulse is partly absorbed in accelerating the mass of the pendulum absorber in the direction of the impulse. Likewise, when the crankshaft relaxes after the impulse, the relaxation is opposed by the inertia of the mass that was accelerated. In order to absorb the rigid-body modes of a crankshaft, a pendulum absorber may be ‘tuned’ by adjustment of its mass and of the distance between its center-of-mass and the rotational axis of the crankshaft. The pendulum absorber may also be tuned via the configuration of its travel path and/or the size of the fulcrum pins that are guided through the travel path.
To provide torsional vibration absorption, a pendulum absorber requires some running clearance between its travel path and one or more slide pins rigidly coupled to the cheek. The inventors herein have observed, however, that during periods of very slow crankshaft rotation—such as engine start or stop—a pendulum absorber may emit an undesirable clunking sound when travelling across the clearance. In other speed/torque regimes, a pendulum absorber may bang against the end-stops at the limits of its intended travel path, also emitting an undesirable sound. In some cases, premature wear of the crankshaft may occur along with the undesirable sound emission.
Some measures have been reported to address the issues identified above. For example, U.S. Patent Application Publication 2011/0031058 to Klotz et al. provides a travel limiter for a pendulum absorber, which is made of an elastomeric material. U.S. Pat. No. 4,739,679 to Berger et al. provides a special caming configuration in addition to elastomeric travel limiters. The inventors herein have observed, however, that elastomeric travel limiters may require frequent replacement when used in a modern motor-vehicle crankshaft. Moreover, the caming configuration of Berger may not fully resolve the noise issues observed at low rotational speeds.
Accordingly, this disclosure provides a series of solutions to address noise and wear of a pendulum-absorber type crankshaft of a motor-vehicle engine. One embodiment is a crankshaft comprising a cheek, a torsion-absorbing pendulum pivotally coupled to the cheek, and a rotational speed actuated brake to oppose motion of the pendulum relative to the cheek. The brake is configured to provide greater opposition to the motion at lower rotational speeds than at higher rotational speeds. In this manner, the greatest braking force becomes available at the lowest rotational speeds—e.g., during engine start or stop—when the pendulum absorber is apt to travel across its clearance, emitting undesirable sound. It will be noted that some of the configurations disclosed herein may also be used for transmission damper pendulum systems.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address the problems or disadvantages noted herein.